The present invention relates generally to a compressor control system for a vehicle air conditioner in order to discharge liquid refrigerant from the compressor, and more particularly to a method of disengaging the compressor clutch automatically during engine start to avoid any engine lock-up or engine damage.
When a vehicle is parked outdoors in the daytime for a considerable time, the passenger compartment is heated by the sun and the temperature becomes higher than the temperature in the engine compartment. If a vehicle is parked for a long time, the above-mentioned temperature difference occurs repeatedly each day, resulting in the refrigerant in the evaporator, which is disposed adjacent to the passenger""s compartment, moving to the compressor which is disposed in the engine compartment at a portion lower than the evaporator and a condenser of the air conditioner system. The refrigerant in the compressor becomes liquid when the temperature falls. When the compressor is operated in this state, the compressor attempts to compress the incompressible liquid which generates an undesirable noise and shock both to the compressor and to the engine which ultimately drives the compressor.
Generally, the pumping torque for a compressor could be expressed as:
Tp=K1xcfx81V2+K2xcex94P
Where V is the fluid velocity which is proportional to the compressor""s RPM. xcfx81 is the fluid density. P is the fluid pressure, and K1 and K2 are constants related to the geometrical and physical parameters of the pump. During engine starts after a long soak period, the second term, K2xcex94P on the right side can be neglected for operating cases just prior to the build up of the pressure difference xcex94P. Since the density of liquid is usually at least 100 times greater than that of gas, depending on the pressure level, the torque needed by the compressor to pump the liquid refrigerant could be at least 100 times higher than that needed to pump the refrigerant in its gas phase. An air conditioning compressor and its drive system are primarily designed for gas phase operations of refrigerant. Therefore, an extremely high reaction torque generated by the compressor as it attempts to displace liquid refrigerant inevitably causes the drive belt to slip and squeal and also provides a shock to the system including the compressor and engine.
From the above equation, it shows that the reaction torque of this special operating condition can only be the same level as to pump the gas phase refrigerant if the compressor is engaged and operated at a speed which is ten times less than the normal operating speed.
It follows that the only opportunity for such low RPM compressor operation could be found during the engine start cranking. In order to provide a compressor control device which discharges the liquid refrigerant from the compressor without significant compression noise, a variable displacement compressor control device is proposed in U.S. Pat. No. 5,617,730 in which a variable displacement compressor is controlled to be engaged at engine start so that the compressor is driven at a limited volume and within a limited number of revolutions of the engine. In so doing, the compressor is purged of the liquid without liquid-compression-noise.
However, in case the compressor is mechanically locked-up, the engine could either not be started or be damaged during the engine start with this device. Some car companies with this device use a lock-up sensor to detect if the compressor is locked-up. If the sensor detects the compressor is locked-up, the engine control device will disengage the compressor. This obviously increases the production cost with an additional sensor.
In view of the above-described circumstances, it is a primary object of the present invention to provide a compressor clutch control device, which disengages the compressor clutch without a dedicated lock-up sensor. Furthermore, this new invention is specifically designed for a fixed displacement compressor with a new control scheme.
Another object of the present invention is to provide a compressor control device that disengages the clutch when the battery voltage drops below a certain limit. When the compressor is locked-up, the starter motor may not rotate or may rotate very slowly with belt slip. When this happens, a high current is drawn from the battery and the battery voltage dramatically drops. Therefore, this control device also maximizes the potential for reliable engine starting when the battery has a low state of charge.
A further object of the present invention is to provide a compressor control device that disengages the clutch when the engine speed is below a certain point after a certain period of time since the engine started cranking. When the compressor is locked-up, the engine may not rotate or may rotate very slowly with belt slip. When this happens, the readings from the engine speed sensor will be low. During normal engine cranking, engine speed gradually increases. The compressor is supposed to be engaged soon after the engine starts to rotate. If, after a predetermined time period, the engine speed has not reached a predetermined speed, the compressor is disengaged.
A still further object of the present invention is to provide a compressor control device that disengages the clutch when the timer reaches a certain point since the compressor is engaged at engine start. During a normal situation, the compressor will be disengaged within a number of limited revolutions. When the compressor is locked-up, the engine cannot rotate or rotates slowly. Therefore, the limit of the revolutions may never be reached. If this lasts too long, the battery or engine assembly could be damaged. This is another way to ensure that the clutch does not stay engaged if the compressor is locked up.
These and other objects of the present invention are obtained by providing a compressor control system for a vehicle air conditioner, comprising a compressor having an input shaft which is selectively driven by a crankshaft of an engine by a clutch member. A control device is provided for controlling engagement of the clutch member at engine start-up to discharge liquid refrigerant from the compressor regardless of a previous operating condition of the vehicle air conditioner. The control device engages the clutch member during engine start-up when the engine coolant temperature is below a predetermined high level, the ambient temperature is above a predetermined low temperature limit and the battery voltage is above a predetermined level. After the clutch member has been engaged, the control device disengages the clutch if a battery voltage drops a predetermined value. The control device also disengages the clutch after clutch engagement if the engine speed exceeds a predetermined high level or falls below a predetermined low level.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.